Field of the Invention
The present invention relates to a magnetostatic wave resonator, and particularly to a magnetostatic wave resonator used, for example, in a band-pass filter or the like.
Description of the Prior Art
FIG. 9 is a perspective view showing a conventional band-pass filter which is a background of the present invention, and is shown in the "Magnetostatic Wave and Magnetostatic Wave-Optic Filter Technology" in MICROWAVE JOURNAL, NOVEMBER, 1990. Though 4-stage magnetostatic wave resonators 1a, 1b, 1c and 1d are used in the band-pass filter shown in FIG. 9, its basis is, for example, a single-stage magnetostatic wave resonator shown in FIG. 10.
The magnetostatic wave resonator 1 shown in FIG. 10 includes a GGG (gadolinium gallium garnet) substrate 2 having a square plane, on one main surface of the GGG substrate 2, a YIG (yttrium iron garnet) thin film 3 is formed, for example, by means of LPE (liquid phase epitaxy). Furthermore, on the YIG thin film 3, two single-wire transducers 4a and 4b are disposed so as to intersect with each other at right angles and to connect magnetically to the YIG thin film 3.
Next, the operation of the magnetostatic wave resonator 1 shown in FIG. 10 is described.
To the YIG thin film 3 of the magnetostatic wave resonator 1, a d.c. magnetic field is applied from the outside, in a direction z shown in FIG. 10 or in a direction intersecting at right angles with the surface of the YIG thin film 3. When a high-frequency signal is inputted to one transducer 4a at this state, a high-frequency electric current flows through the transducer 4a and a high-frequency magnetic field is excited around thereof. By the high-frequency magnetic field, a magnetostatic wave is excited in the YIG thin film 3. At this time, the magnetostatic wave starts to resonate at a frequency decided by the size, the thickness and the saturated magnetization of the YIG thin film 3 and the intensity of the d.c. magnetic field applied thereto. The other transducer 4b converts the magnetostatic wave excited in the YIG thin film 3 inversely into a high-frequency signal and outputs the high-frequency signal. Thus the magnetostatic wave resonator 1 operates as a resonator.
In the magnetostatic wave resonator 1, however, as showing its frequency characteristic in FIG. 11, not only a main mode but also an unnecessary higher mode is excited. The excitation of the unnecessary higher mode is due to the resonance of higher standing wave of the magnetostatic wave, thus when the magnetostatic wave resonator 1 is operated as a filter, the attenuation quantity outside a passband is deteriorated.
As means for suppressing the unnecessary higher mode, a magnetostatic wave resonator wherein the higher mode is suppressed is disclosed in "Magnetostatic wave resonator using microstrip disk" of C-99 at all Japan Spring Session 1991 of Japan Electronic Information and Communication Association.
FIG. 12 is a perspective view showing the conventional magnetostatic wave resonator wherein the higher mode is suppressed. In the magnetostatic wave resonator 1 shown in FIG. 12, on a surface of a YIG thin film 3, a metal disk 5 is formed, for example, by means of etching. In the magnetostatic wave resonator 1, a magnetostatic wave energy is confined by the metal disk 5 in the YIG thin film 3 thereunder, thereby operating as a resonator. In this case, in the magnetostatic wave resonator 1, the confining operation is weak against the higher mode, as a result, the higher mode is suppressed.
In the magnetostatic wave resonator 1 shown in FIG. 12, however, as showing its frequency characteristic in FIG. 13, though the higher mode is suppressed than that of the magnetostatic wave resonator shown in FIG. 10, a higher mode level is still high.
On the other hand, when neglecting an increase of insertion losses, though it is possible to suppress the higher mode, usually the increase of insertion losses is not desirable.